Itsuki was a very unique villain on the show.
He was originally a demon who had spied upon Shinobu Sensui. Shinobu spared his life at the last minute.
And not long after that, he often accompanied Shinobu Sensui on his Spirit Detective missions.
Many years later, he joined him on his quest to open up a tunnel to Demon World.
And it was his magical power that allowed the portals to open and form.
His magic power wasn’t limited to portal opening.
He was able to trap Kurama, Hiei, and Kuwabara in another dimension.
But they were able to free themselves when Kuwabara opened up his magical power with his new sword that could cut through dimensions.
After Shinobu Sensui died, he took his body and soul to a dimension free from Spirit World or Demon World. He was openly gay for him, which makes a lot of sense. It is unclear if the feelings were mutual between the two. I would say it’s anybody’s guess.